How to use BJT as a switch?

posted on July 13, 2019

During lecture on transistor modes, we discussed transistor works in different modes subject to change in current at the base. In some applications BJT works in one mode and in another application transistor works in another mode. For example transistor as an amplifier works in active mode.

When we want to use transistor as a switch we have to operate it between saturation and cutoff mode. We also discussed during lecture onBJT that base plays a pivotal role in the transistor working. If we change current on the base, we can switch transistor from one mode to another.

Transistor As A Switch Circuit Diagram

The following figures 1 and 2 explains the working of transistor switch.

Cutoff Mode Situation in the Circuit

Fig-1: Cutoff mode condition

In fig-1 the transistor configuration is common emitter and this is NPN transistor. As we know in cutoff mode the base-emitter junction is reverse biased. So base voltage must be less than or equal to emitter voltage in case of an NPN transistor. In the above circuit if VBB =0, then there would be no current flow through the transistor. In other words IB = 0 as well as IC is also equal to zero. Under this condition transistor behaves as an open circuit.

So mathematically this situation can be described as below

IB=0 or VBB = 0 ⇒ IC=0

And

VCE= +VCC

Saturation Mode Situation in the Circuit

Fig-2: Saturation mode condition

As we know in saturation mode both emitter-base as well as the base-collector junctions are forward biased. There should be enough VBB or IB to provide maximum collector current i.e IC(sat) which put transistor in saturation mode. i.e ignoring VCE which is very small

IC(sat)=VCC/RC

Fig-3: Switching operation Regions

If you observe in the above figure that there should be minimum IB required to produce IC(sat) i.e

IC(sat)=IB(min)

Applications of BJT Switching Operation

Lamp Switch ON-OFF

A BJT can be used to switch the light on and off. A square wave is applied at the base of transistor. When the square wave is at lower level lets say 0V. The transistor is in cutoff mode. When the square wave is at higher level it forward biases the base-emitter junction and hence the transistor goes into saturation. (See fig-4)

Fig-4: BJT used to switching a lamp

Fire Alarm System

Transistors can be used in fire alarm systems. A thermistor is connected at the base of the transistor. The resistance of thermistor decreases with increase in temperature. So when there is a fire in the surroundings the temperature increases which results in decrease of thermistor resistance. So enough current flows from Vbb to the base of the transistor which forward biases the base-emitter junction. Hence transistor goes into saturation and the buzzer starts. (See fig-5)

Fig-5: BJT in fire alarms system

Automatic Street Light

Automatic street light is another application of BJT. A light dependent resistor (LDR) is used. The resistance of LDR is very high. During day time when LDR has very low resistance. Almost all voltage drops across LDR. In this case potential at base of NPN transistor is almost equal to emitter i.e 0. So BJT will be off.

During Night time When LDR resistance increases, potential drop across LDR is almost zero. So voltage at the base is equal to +V1. The transistor will be ON and hence lamp starts glowing. (See Fig-6)

Fig-6: Transistor application in street light

Advantages of transistor as a switch

You need a very small amount of current to switch ON the transistor.

Transistor itself handles most of the load current

Transistor switch is fast

Transistor is low cost and small size

They produce zero noise as compared to relays

More reliable as compared to electromechanical switches

Disadvantage of transistor as a switch

They can be used to switch ON and OFF to DC loads only

Temperature can affect the switching operation

Only use in low voltage and current applications

Transistor vs Relay; What to use?

Everything has some pros and cons. It totally depends upon your application and cost of the project. But below I make some comparison points to guide you which one is suitable for you. I mean transistor or relay.

Relays can work with AC whereas BJT works with DC loads only

Relay has infinite resistance in case of open circuit. On the other side transistor has some leakage collector current.

Relays are used in heavy loads but transistor is used for small loads.

Relay is slower than transistor.

Lifetime of a relay is much shorter than semiconductor devices.

Relays can operate at very high temperatures but transistor’s performance can be impacted by high temperature usually more than 95 Celsius.